Valorizing Waste Fly Ash Cenospheres: Towards Multifunctional PLA Composites With Improved Flame Retardancy and Thermal Insulation

To valorize industrial waste and enhance polylactic acid (PLA), this study hypothesizes that functionalizing hollow fly ash cenospheres (FAC) with a phosphorus-nitrogen system creates a synergistic flame-retardant and thermal insulating additive. Accordingly, a multifunctional filler (FAC-KH550-PA-MEL) was synthesized and characterized via Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). PLA composites were subsequently evaluated using thermogravimetric analysis (TGA), limiting oxygen index (LOI), cone calorimetry, and thermal conductivity measurements. Results demonstrate that 5 wt% additive enabled PLA to achieve a UL-94 V-0 rating and an LOI of 31.8%, with a 28.06% reduction in peak heat release. These improvements stem from a synergistic mechanism: phytic acid (PA) promotes condensed-phase charring, melamine (MEL) provides gas-phase dilution, and the silicate skeleton of FAC acts as a thermal barrier. Furthermore, the unique hollow structure of FAC reduced thermal conductivity by 36.25% to 0.0712 W/(m K). This confirms that low-loading functionalized FAC effectively yields high-performance, multifunctional PLA composites.